1. Field of the Invention
This invention relates to photographic supports and elements, and more particularly, to photographic supports and elements comprising a paper base material having thereon a polyolefin coating containing a white pigment and an optical brightener.
2. Background of the Invention
A valuable class of photographic supports and elements comprises a paper base material having thereon a polyolefin coating containing a white pigment and an optical brightener. Such supports are particularly useful in the preparation of photographic elements such as color prints because they exhibit good brightness and excellent dimensional stability and are highly resistant to the action of aqueous acid and alkaline photographic processing solutions. The polyolefin coating provides a very smooth surface which is desirable when thin layers, such as silver halide emulsion layers, are to be coated thereover, U.S. Pat. No. 3,411,908 describes such a support which has achieved widespread commercial acceptance.
The purpose of the optical brightener is to make the white areas of the support appear even brighter. The optical brightener fluoresces upon irradiation with ultraviolet (UV) light, emitting visible light, usually bluish in hue, thus enhancing the brightness of the support. Optical brighteners for use in photographic print materials must absorb UV light, especially in the region from 360 to 420 nm, and reemit such light so as to enhance the brightness of the print, and have the desired brightening power. The optical brightener must also be stable to the temperatures, as high as 310.degree. to 330.degree. C., used in incorporating it into the polyolefin and in extruding the polyolefin onto the paper base material.
A problem that has developed in employing optical brightening agents in the polyolefin layer for photographic elements is that optical brightening agents have a tendency to migrate toward the surface of the polyolefin and exude from the surface to form a film on the surface of the polyolefin. Such exudation not only gives rise to a nonuniform brightness of the reflection surface of the support, but also readily transfers to any other surface contacted with that surface. For example, optical brightener transferred nonuniformly to the backside of the adjacent layer of support when wound in roll form can adversely affect subsequent coating and finishing operation and, in consequence, the quality and performance of the final photographic element.